1. Field of the Invention
The present invention relates to a CIS (Contact Image Sensor) flatbed scanner, especially to a CIS scanner having an improved structure for a CIS module that can assure a document sheet in focus and reduce the size of the scanner.
2. Description of the Prior Art
A conventional flatbed scanner preferably uses a CCD line sensor for reading the image information from a sheet because the CCD line sensor can provide stable and good picture quality of images. The trade-off for using a CCD line sensor is that the size of the flatbed scanner is restricted by the light path required for the CCD line sensor. For this reason, the size of the flatbed scanner can hardly be reduced in a great scale even with the aid of mirror arrays to reflect the light beams.
Recently, a new approach is trying to replace a CCD line sensor with a CIS contact sensor for producing low-end flatbed scanners. An advantage for this new approach is that it can save a lot of time and efforts in assembly and maintenance because a CIS contact sensor is modularized. All the necessary optical elements, such as mirrors, light source and a photoelectric converting device, are included in a compact module. Thus, a CIS module can help to simplify the inner structure of a scanner. Moreover, a CIS contact sensor is smaller and lighter than a CCD line sensor. With a CIS contact sensor, the scanner can be portable with height around 30 mm.
Nevertheless, this new approach did not prevail. The main reason is because the scene depth of a CIS contact sensor is only about 0.3 mm. Based on the structure of a conventional flatbed scanner, it is very difficult to assure a document sheet in focus. Even a small manufacture error or structure distortion will affect the picture quality of the images.